Twenty-two chimpanzees sit quietly in the enclosure. The atmosphere is one of calmness. On the left a few females are grooming one another, on the right children are playing. It is not at all obvious that there is something going on, in spite of the quietness. Two males sit three meters apart from one another. A third male is looking at them from a distance. He has his hairs on end and sways ever so lightly with his upper body. Only when he gets up and walks towards the other two males, with deliberately restrained gait, doeshe draw attention to him himself. He jumps over one of the two males. The male concerned bends over and makes a characteristic oh-oh sound, followed by a short scream, but remains seated. The presentation continues, for a full quarter of an hour. The male hoots, stamps his feet, slaps the ground, hits a few females and jumps over the other male two more times. The other male again makes the oh-oh sound and screams, but remains seated. The third male does not move and does not make a sound. At last the fuss-pot seems to give up. He starts grooming some women. After a quarter of an hour he begins anew with his display. Now he jumps over both males, two times over the one, once over the other. Both make the oh-oh sound and stay where they are. After some ten minutes the show is over. Our male sits down and after a while walks in another direction. A few minutes later one of the seated males gets up and quietly walks away.

Separating Interventions between male chimpanzees

(Otto Adang)

The chimpanzee male who made such a fuss was Nikkie. The pictures show him performing the separating intervention described above (Nikkie on the left, Yeroen in the middle, Luit on the right- bending forward and submissively greeting Nikkie who jumps over him). At that time he had been the alpha male of the group of chimpanzees in Arnhem Zoo for two years. All others in the group behaved subordinately towards him. Nikkie himself was subordinate to no-one. He looked impressive. His hairs were always a little erect, which made him look bigger than he actually was. Whenever he displayed, his hairs would be fully erect and he would stamp his feet and slap the ground. Other chimpanzees would get out of his way and show their submissiveness to him by making a panting oh-oh sound. If they did not get out of his way soon enough, they would get hit.

Balance of power

At first sight, Nikkie was the unassainable leader. Yet there was something lacking, although you would only notice that if you looked for a longer time. During conflicts, the females in the group did not support him. They would rather support Yeroen, one of the other males. The women also seemed to behave more respectful towards Yeroen than towards Nikkie. In spite of that, Yeroen was not the alpha male. He had been alpha in the past, but he had been unable to maintain that position. Yeroen was clearly subordinate to Nikkie, but still held a key position. Nikkie's status as the most powerful male of the group was due to a coalition with Yeroen. Nikkie and Yeroen often acted together. They usually sat next to each other and generally displayed jointly. In conflicts with other group members, they would support one another. Through their coalition, Nikkie and Yeroen kept the third male, Luit, at bay. Luit seemed to be the strongest of the three males, but that was not enough. Sometimes he would start a display, but Nikkie tended to put and end to that by intimidating him. Nikkie's partnership with Yeroen was too much for Luit. The fourth male, Dandy, did not take part in the power games of the other males. He usually sat alone or next to some women. He seldom displayed and no-one was submissive to him. He did not interfere in quarrels between others.

Testing the coalition

Whereas one of the functions of separating interventions may be to prevent the formation of a rival coalition, another function might lie in the testing of the existing coalition. Nikkie's veto on contacts between Luit and Yeroen could only be adhered to if Yeroen preferred a working relationship with him above contact with Luit. Every succesful intervention underlined the succes of the coalition, but also made clear that Nikkie depended on Yeroen's willingness to comply. Most of the time either Luit or Yeroen left the scene when Nikkie started one of his separating interventions, but it never was a matter of course.

For, in the end, Nikkie could not coerce the other two males to keep their distance from one another. If he tried too hard, he risked an attack. By Luit, by Yeroen or by both. It was rare, but it did occur a few times that Luit and Yeroen acted together. E.g. on one occasion when Nikkie slapped Luit while jumping over him. Yeroen jumped to his feet and, screaming, chased Nikkie. Luit followed suit. It did not result in a physical altercation, but Nikkie screamed and took refuge with two females.

Prevention

A change in this apparently stable situation could only occur if Luit would have an alternative for the partnership between Nikkie and Yeroen. On his own, it was impossible for Luit to challenge the other two succesfully. For a coup, he would need help. Maybe the women's collective could provide assistance. Or Dandy could become an active player in the power game. Or Yeroen could change alliances. But none of that happened. Luit was a potential rival and not an active challenger. Nikkie seemed keen to keep it that way and he diligently saw to it that Luit was unable to forge an alliance with someone else. Separating interventions were a means to prevent Luit from forming a bond with others. Nikkie reacted highly selectivel on contacts between Luit and Yeroen. The long-term effect was that Luit and Yeroen were hardly ever in contact with one another. The contact frequency between Yeroen and Luit was much less than that between Luit and Nikkie and that between Nikkie and Yeroen. It was next to impossible for Luit and Yeroen to be close to one another without Nikkie being around. Mostly displaying, sometimes as third partner. Nikkie also disrupted Luits contacts with high ranking females. On those occasions, Yeroen acted as Nikkies assistant and as instigator of interventions. He used to point out "forbidden" contact to Nikkie by grunting softly or by going over to Nikkie and touching him.